1. Field of the Invention
This invention relates to downhole testing apparatus having pumps with pressure limiters for pumping fluid to inflate inflatable packers, and more particularly, to a pressure limiter in which fluid pressure in the tool is not vented to the well annulus during an actuation cycle.
2. Description of the Prior Art
A known method of testing a well formation is to isolate the formation between a pair of inflatable packers with a flow port therebetween adjacent the formation. The packers are inflated by means of a pump in the testing string which pumps well annulus fluid or mud into the packers to place them in sealing engagement with the well bore.
Typically, positive displacement pumps are used. One such downhole pump is actuated by the vertical reciprocation of the tubing string connected to the pump, such as disclosed in U.S. Pat. No. 3,876,000 to Nutter and U.S. Pat. No. 3,876,003 to Kisling, III.
Other pumps are actuated by rotation of the tool string. U.S. Pat. No. 3,439,740 to Conover and U.S. Pat. No. 4,246,964 to Brandell, both of which are assigned to the assignee of the present invention, disclose a rotationally operated pump having a plurality of vertically reciprocating pistons which are driven by a cam structure. Inlet and outlet valves are positioned adjacent each of the pistons.
A simpler, sleeve-type pump piston is used in the downhole pump of Evans et al., U.S. Pat. No. 3,926,254, assigned to the assignee of the present invention. In the Evans et al. apparatus, as well as the other pumps described above, the pump piston is in direct contact with the well annulus fluid.
The downhole pump described herein for use with the pressure limiter of the present invention includes a single sleeve-type pump piston, but further includes a diaphragm which separates a piston chamber in which the piston reciprocates from a pumping chamber with inlet and outlet valves therein through which the fluid is moved to inflate the packer. The piston chamber is filled with a clean hydraulic lubricant which promotes longer life for the pump parts. Backup piston wiper rings are provided to clean the piston of abrasive particulate in the event the diaphragm is ruptured.
Simple inlet and outlet check valves with resilient annular sealing lips are used, and these valves are not easily clogged or damaged by abrasives in the well fluid. These valves are similar to valves in the Halliburton Omni RS Circulation Valve, assigned to the assignee of the present invention and described in co-pending Application Ser. No. 797,375.
When inflating the packers, it is essential that the packers not be overinflated and damaged. To accomplish this, most of the pumps of the prior art include relief valves which relieve pressure from the pump to the well annulus. A major problem with such devices is that if the relief valve is stuck in an open position, the pump cannot be used to inflate the packers and complete an operation. A pump without a relief valve is disclosed in U.S. Pat. No. 4,313,495 to Brandell, assigned to the assignee of the present invention. In this pump, a clutch is used which is disengaged when the pump pressure reaches a predetermined level, thus making the pump inoperative.
The pressure limiter of the present invention limits packer pressure internally and does not vent fluid therein directly to the well annulus. In a first embodiment, the pressure limiter vents around the outlet check valve to the packers at the lower end of the testing string.
In a second and third embodiment, a piston reciprocates in the pressure limiter when the packer pressure reaches the desired level. This reciprocating piston increases the pumping chamber volume in response to the displacement of the pump. As with the first embodiment, there is no venting to the well annulus.
In a fourth embodiment, the pressure limiter is not a separate component, but instead is characterized by the pumping chamber being of predetermined size. As the differential pressure across the pump increases, the efficiency gradually decreases. By proper sizing of the pumping chamber, the efficiency becomes essentially zero at the desired pressure. Therefore, further operation of the pump will not further increase the pressure.
While the pressure limiter of the present invention is adapted for use with the diaphragm pump described herein, it should be emphasized that the pressure limiter could be used equally well with any positive displacement pump, and the invention is not intended to be limited to any particular pump configuration.